Triacetylcellulose (TAC, also called cellulose triacetate) film has traditionally been used by the photographic industry due to its unique physical properties and flame retardance. TAC film is also the preferred polymer film for use as a cover sheet for the polarizers used in liquid crystal displays. It is the preferred material for this use because of its extremely low in-plane birefringence. Its out of plane birefringence is also small (but not zero), and is useful in providing some optical compensation to the LCD.
Intrinsic birefringence describes the fundamental orientation of a material at a molecular level. It is directly related to the molecular structure (bond angles, rotational freedom, presence of aromatic groups, etc.) of the material. The intrinsic birefringence is not affected by process conditions (temperature, stresses, pressures) used to make a macroscopic object.
Crystalline and liquid crystalline materials have the convenient property that their intrinsic birefringence manifests itself almost perfectly when they are assembled into a macroscopic article. Layers of crystalline and liquid crystalline molecules often can be manufactured such that all the molecules in the article are in registry with each other and thus preserve their fundamental orientation. The same is not true when making layers of an amorphous polymeric material. Their intrinsic birefringence can be highly modified by the manufacturing process. Thus, the measured birefringence of an actual article will be a resultant of its intrinsic birefringence and the manufacturing process. Because in some embodiments we are dealing with such amorphous polymeric materials, the following definitions refer to this measured birefringence and not intrinsic birefringence.
In-plane birefringence, Δnin, means the difference between nx and ny, (nx−ny), where x and y lie in the plane of the layer, and where nx and ny are indices of refraction in the directions of x and y, respectively. Here, the x axis is taken as a direction of maximum index of refraction in the x-y plane and the y direction is perpendicular to the x axis. Accordingly, nx will be defined as always being the larger refractive index, and ny will be defined as the being the smaller refractive index and in the y direction, perpendicular to nx. The sign convention used will be nx−ny and will always be positive.
In-plane retardation, Rin, is a quantity defined by (nx−ny)d, where d is a thickness of the layer in the z-direction, perpendicular to the x-y plane. Rin will always be a positive quantity. The values of Δnin and Rin hereafter are given at wavelength λ=590 nm.
Out of-plane retardation Rth, of a layer is a quantity defined by [nz−(nx+ny)/2]d, where nz is the index of refraction in z-direction. The quantity [nz−(nx+ny)/2] is referred to as out-of-plane birefringence, Δnth. If nz>(nx+ny)/2, Δnth is positive, thus the corresponding Rth is also positive. If nx<(nx+ny)/2, Δnth is negative and Rth is also negative. The values of Δnth and Rth hereafter are given at λ=590 nm.
Amorphous means a lack of molecular order. Thus an amorphous polymer does not show molecular order as measured by techniques such as X-ray diffraction.
Chromophore means an atom or group of atoms that serve as a unit in light adsorption. (Modern Molecular Photochemistry Nicholas J. Turro Editor, Benjamin/Cummings Publishing Co., Menlo Park, Calif. (1978) Pg 77). Typical chromophore groups include vinyl, carbonyl, amide, imide, ester, carbonate, aromatic (i.e. heteroaromatic or carbocylic aromatic such as phenyl, naphthyl, biphenyl, thiophene, bisphenol), sulfone, and azo or combinations of these groups.
Non-visible chromophore means a chromophore that has an absorption maximum outside the range of 400–700 nm.
Continuous means that articles are in contact with each other. In two contiguous layers, one layer is in direct contact with the other. Thus, if a polymer layer is formed on the substrate by coating, the substrate and the polymer layers are contiguous.
Synthetic polymer films (such as polycarbonate or polysulfone) are often used to enhance the minimal optical compensation that TAC provides. These synthetic polymers films are attached to the rest of the display by adhesive lamination.
Generally in the field of optical materials, the synthetic polymer film is used as an optically anisotropic film (having a high retardation value), while a TAC film is used as an optical isotropic film (having a low retardation value).
European Patent Application No. 0911656 A2 and Japanese Patent Publication 2000/275434 A both disclose a TAC film having high retardation. The TAC is used as a support for an optical compensator sheet, which comprises the TAC support and an optically anisotropic layer containing a discotic liquid crystal molecule. The TAC film achieves high retardation by three methods (including the combination of these three methods): 1) the addition of special aromatic small molecules (i.e. triphenylene) to the TAC film, 2) cooling of the TAC solution before casting the film, and 3) stretching the TAC film. The addition of special aromatic molecules is discussed as being problematic as it can lead to “bleeding” of these molecules out of the TAC film. Also in the examples of this invention very long times (over an hour) are required to dry such TAC films. Such times would not be amenable to a roll to roll process.
In addition to the TAC film, the highly anisotropic, discotic liquid crystal layer requires a special alignment technique and ultra violet radiation to crosslink this monomeric layer.
U.S. Published Patent Application 2001/0026338 A1 discloses a single TAC film with high retardation without the highly anisotropic discotic layer. This TAC film achieves high retardation by the incorporation of molecules with two or more aromatic groups into the TAC film followed by stretching of the TAC film. Without such stretching, this TAC film does not demonstrate any enhanced retardation compared to regular TAC. With this stretching both in and out of plane retardation are increased. These two orthogonal retardations cannot be independently controlled by this method.
Japanese Published Patent Application JP1999-95208 describes a liquid crystal display having an optical compensator (having high retardation) prepared by uniaxial stretching of a high polymer film. Such polymers include polyesters, polycarbonate, or polysulfone. This stretching step is essential to obtain the desired optical properties. This stretching affects both in- and out-of-plane retardation simultaneously. These two orthogonal retardations cannot be independently controlled by this method. Also, producing uniform optical compensators by this method is described as being difficult.
This application also describes a compensator where the inventor uses an exfoliated inorganic clay material in a polymeric binder coated on top of a TAC support. The exfoliated inorganic clay material in this layer is the optically active material, not the polymeric binder.
Japanese Published Application JP2001-194668 describes a compensator made by laminating polycarbonate films that have been stretched. Not only does the approach require lamination (with its associated difficulties), but it also requires two independent stretchings of two different types of polycarbonate. The lamination step also requires that the two films be in registry with each other and that their optical axes be orthogonal to each other.
U.S. Pat. Nos. 5,344,916, 5,480,964, and 5,580,950 describe compensation films for LDCs. However they do not mention the need for barrier layers to control curl and improve adhesion.
It is a problem to be solved to provide a multilayer optical compensator that is readily manufactured, that provides the required degree of in-plane and out-of-plane compensation, that has excellent adhesion between layers and is free of curl caused by application of organic solvent coating solutions.